Intermolecular Forces
How are molecules held together?
There are two types of attraction in molecules:
◦ Intramolecular forces
◦ Intermolecular forces
Intermolecular Forces – the forces that
bond molecules to each other
Intramolecular Forces – the forces that
bond the atoms to each other within a
molecule
Until now, we have been discussing
intramolecular forces (ionic, polar, non-polar
bonds)
How are molecules held together?
Intermolecular forces are weak relative to
covalent bonds
Intramolecular forces are strong covalent
bonds
We can conclude that it does not take very
much energy to break the bonds that hold
molecules to each other (intermolecular
forces)
These forces are also referred to as van
der Waals forces after the physicist who
studied them
We will be discussing 4 types of
intermolecular forces
◦
◦
◦
◦
Dipole-Dipole Forces
Ion-Dipole Forces
Dispersion (London) Forces
Hydrogen Bonding
Dipole-Dipole Forces
Dipole is a polar
molecule
These molecules will
orient themselves so that
oppositely charged ends
of molecules are near to
one another
The electrostatic
attractions between
these ends are called
dipole-dipole forces
+
+
–
H
Cl
–
+
–
Lets see it!
Result of Dipole-Dipole Forces
Polar molecules will tend to attract one
another more than similarly sized non-polar
molecules
Therefore the energy required to separate
polar molecules from one another is
greater than that needed to separate nonpolar molecules
Ion-Dipole Forces
The force of attraction between an ion and
a polar molecule (a dipole)
Example: NaCl breaks up because the ion
dipole with water is stronger than the
attraction of Na+ to Cl-
Let’s See it!
Dispersion (London) Forces
Weakest intermolecular force that act
between non-polar molecules
Shared pairs of e- can temporarily occupy
positions that create momentary, uneven
distributions of charge
This causes non-polar molecules to
become slightly polar for just an instant
At this instant it is
capable of inducing a
dipole in a nearby
molecule resulting in
an intermolecular
force of attraction
This is called a
dispersion force
Hydrogen Bonding
An attraction between the hydrogen atom,
bonded to a highly electronegative atom,
and the negative end of a dipole nearby
Like Dissolves Like
Ionic solutes dissolve in polar solvents
◦ (ex: NaCl and H2O)
Non polar solutes dissolve in non polar
solvents
◦ (ex: solid I2 and liquid Br2)
Practice
Sheet on polar molecules is for homework!
On this sheet please add this question:
Based on what we know about
intermolecular forces predict the
solubility of this molecule in a) water
and b) acetone
0
